If at an incident a casualty is not breathing, the immediate and crucial response is to call emergency services (such as 999 in the UK) and start cardiopulmonary resuscitation (CPR) right away. Begin by checking the scene for safety, then assess the casualty’s responsiveness. If unresponsive, open their airway by tilting the head back and lifting the chin to clear the airway. Look for normal breathing by observing chest rise without putting your face close to theirs. If they are not breathing normally, give chest compressions in the center of the chest at a rate of about 100 to 120 compressions per minute. If trained and comfortable, provide rescue breaths as well, in cycles of 30 compressions followed by 2 breaths. Continue CPR until professional help arrives or the person begins to breathe normally. If available, use an automated external defibrillator (AED) as soon as possible following its prompts. This immediate intervention is vital to maintaining blood flow and oxygen to the brain and vital organs, significantly increasing the casualty's chances of survival.